Flat-panel display devices such as liquid crystal display devices and organic electroluminescent display devices have been utilized in various fields of office automation (OA) devices, information terminals, clocks, television receivers, etc., with features of lightness, thinness, lower power consumption, etc. More specifically, active-matrix display devices using thin-film transistors (TFTs) have been utilized as monitors for displaying large amounts of information, such as mobile devices and computers, with their fast response.
In the active-matrix display devices, thin-film transistors are utilized for pixel switching elements. The thin-film transistors are also utilized for switching elements of drivers formed in a frame region (non-display area) outside an active area (display area). Semiconductor layers of the thin-film transistors are formed of amorphous silicon, polycrystalline silicon or oxide semiconductor.